1. Field of the Invention
Methods for the preparation of aliphatic alcohols having from 2 to 5 carbon atoms from the corresponding olefins are well known. Broadly, an enriched olefin feedstream is reacted in the presence of an acidic reacting catalyst at an elevated temperature to produce a reaction product containing either a saturated aliphatic alcohol or its ester which can be hydrolyzed to the alcohol. In earlier processes, sulfuric acid was employed as the preferred catalyst. This particular type of reaction resulted in the formation of an ester of the alcohol as an intermediate reaction product. The alcohol ester was then hydrolyzed to form the alcohol and a dilute aqueous solution of sulfuric acid. Serious drawbacks to this esterification process was the corrosivenesss of the sulfuric acid catalyst to process equipment and the large energy input required to return the sulfuric acid to a purity and a concentration that permitted its reuse in the process.
Next, processes for the direct hydration of an olefin with water to produce an alcohol were developed. Direct hydration involves contacting an olefin feed stream and water with a solid catalyst having acidic reaction sites. Examples of earlier solid catalysts for this reaction were sulfuric acid deposited on silica gel or clay, and reduced tungsten oxide. A major problem experienced with the early direct hydration processes was that they led to the production of substantial amounts of undesired olefin polymer and ether by-products. Another feature which led to serious problems was the necessity to employ a high reaction temperature. Under the high temperature reaction conditions, the solid catalysts had inadequate stability and possessed a short catalyst life.
There is another class of solid acidic reacting catalysts whose development arose out of chemical processes unrelated to the direct hydration process for olefins. These are the ion exchange resin catalysts. Ion exchange resins came into use for decationization and water purification. Applications for these catalysts broadened leading to their use in metal recovery processes and in food and pharmaceutical industry processes. The synthetic sulfonated ion exchange resins were then proposed as catalysts for the direct hydration of olefins to the corresponding alcohol.
The ion exchange resin catalysts can be defined as synthetic aryl resins possessing a hydrocarbon skeleton combined with strong mineral acid groups. They comprise the solid cross-linked polymers of vinyl aromatic compounds, such as styrene or vinyl polymers, divinyl benzene and other monoethylenically unsaturated compounds. These catalysts can be prepared in a number of ways. In the case of a polystyrene type resin, a mixture of styrene and divinyl benzene, in which the divinyl benzene may constitute from about 4 to 20% of the mixture, is polymerized at an elevated temperature preferably in the presence of a catalyst to produce a solid polymeric resin. The product resin is conveniently produced in the form of a sphere. The resin is contacted with a free acid, such as sulfonic acid or sulfuric acid, and this reaction is conducted until many or all of the available reaction sites in the resin have taken on a sulfonic acid group. The general method for preparing the acid reacting ion exchange resin catalyst is well known and is not part of the present invention.
Since the ion exchange resins catalysts are totally synthetic materials, considerable latitude exists for their manufacture. Thus, a manufacturer may employ a number of olefinic monomers and utilize them in a broad range of proportions in the polymerization step leading to the linking or cross-linking structure of the resin. In addition, the actual polymerization of the olefin monomers is greatly influenced by the resin polymerization catalyst employed and by the conditions of polymerization including the temperature and duration of the reaction and the use of a solvent or emulsion for effecting the polymerization. Because of the number of variables involved in effecting the preparation of the catalyst, a broad range of ion exchange resin catalysts have been produced exhibiting markedly different physical structures and physical properties.
The physical structure of the ion exchange resin is a vital characteristic of the resin and has a material effect on the usefulness of this class of catalyst. The physical structure of the catalyst not only determines the amount of surface area that is available for effecting chemical reaction but also has a pronounced effect on the stability and the effective life of the catalyst.
Ion exchange resin catalysts are known to be subject to swelling in the presence of solvents. Thus, organic solvents such as benzene, toluene, xylene, carbon tetrachloride and trichloroethylene promote swelling and materially alter the physical structure of the ion exchange resin. Young, U.S. Pat. No. 3,813,908, teaches that granulated styrene-divinyl benzene hydrocarbon copolymers can be swelled by contact with 10 to 50 volume percent of tetrachloroethylene to as much as about 170% of the original copolymer volume. He also discloses that this swelling influences the subsequent disintegration characteristics of the polymer and that the use of a careful or moderate swelling technique can be effective for reducing catalyst disintegration or instability.
The management of the swelling characteristics of the ion exchange resin is also very important during the sulfonation reaction when the active acid groups are added to the available cites in the resin. The art teaches that a too rapid dilution of the resin with water tends to weaken the resin structure and can result in subsequent fracture of the catalyst granules.
Because the resin structure is fragile, it is essential to maintain a high water content in the catalyst during storage and until its introduction into a conversion process. Any undue loss of water content during this period can reduce the catalytic activity of the resin as well as its physical strength thereby leading to early disintegration of the granules upon subsequent contact with water. When a partially dried out resin is placed in water, the water absorption may be so rapid that severe disintegration of the granules takes place. This Young teaching points to the destabilizing elements that are factors in the manufacture, storage and in the handling of ion exchange resin catalysts. These destabilizing features, which are in addition to the substantial differences in the structures of the catalysts, further complicates the problem of selecting the most stable, long-lived catalyst for an olefin hydration process.
Work has been conducted to distinguish or classify the ion exchange resin catalysts according to their physical structure. Thus, certain of the ion exchange resin catalysts have been described as having a gel-type or microporous resin structure. Commercial ion exchange resin catalysts which have been disclosed as having a gel type structure include: Amberlite 120, Amberlite 124 and Dowex 50X8. Another class of ion exchange resin catalysts have been described in the technical and commercial literature as having a macroporous resin structure. Examples of macroporous ion exchange resins include Amberlyst No. 15, Amberlite 200 and Lewatit SPC 118/H. Members of both of these classes of ion exchange resins have been disclosed as useful in the olefin hydration process for alcohol production.
The classification work on ion exchange resin catalysts up to the present time is limited in extent. In general, the published physical characteristics of the resins have been determined under a single set of test conditions. Structural measurements taken under one set of conditions, however, are incapable of gauging the range of characteristics and structural changes in a resin that are brought about by swelling of the catalyst. As pointed out, this swelling is an inherent property of ion exchange resins and its occurrence exerts a vital influence on the effectiveness and the stability or catalyst life of the resin. The paucity of available physical characteristics on ion exchange resins under different conditions of swell or expansion and the absence of any known criteria on which to determine the ion exchange resin catalysts having the greatest stability and a remarkable ability to retain the sulfonic acid catalyst function leaves the art without any rule on how to select a superior catalyst for an olefin hydration process.
2. Description of the Prior Art
German Patent Application No. 1 210 768 discloses a process for the continuous preparation of isopropanol and diisopropyl ether by the catalytic hydration of propylene. The catalyst employed is a strongly acidic cation exchange resin consisting of a styrene polymer cross-linked with from about 5 to 20 weight percent of divinyl benzene containing approximately one sulfonic acid group per aromatic ring. The reaction conditions employed to produce alcohol as the main product include a pressure ranging between 17 and 105 atmospheres, a temperature ranging from about 135.degree. to 157.degree. C. and a mole ratio of from 4 to 10 moles of water per mole of propylene. The feed rates for this process range from 0.5 to 10 volumes of liquid propylene per volume of wet catalyst per hour which corresponds to about 6.7 to 123.4 moles of propylene per liter of catalyst per hour. This process provides a per pass conversion of from 20 to 90 moles of charged propylene with a conversion of about 35 percent being preferred. Under these conditions the optimum selectivity for isopropanol was reached at a temperature of 135.degree. C., amounting to 69 mole percent of the propylene charge of which 22 mole percent were converted, the balance being by-products, namely 28 moles percent diisopropyl ether and 3 mole percent of propylene polymers.
U.S. Pat. No. 2,813,908 discloses a process in which the catalyst employed in a sulfonated copolymer consisting of about 88 to 96 percent styrene and from 12 to 4 percent of p-divinylbenzene, and containing from 12 to 16 weight percent of sulfur in the form of sulfonic acid groups. This patent discloses reaction temperatures from 120.degree. to 220.degree. C. a feed rate of from 0.5 to 1.5 volumes of liquid olefin per volume of catalyst per hour and a water to olefin mole ratio ranging from 0.3 to 1.5. This reference also shows that good selectivity for isopropyl alcohol is achieved at a low temperature, i.e., about 120.degree. C., and at a low conversion of about 3.9 mole percent. When a higher temperature (170.degree. C.) was employed the conversion of propylene rose to about 35 mole percent but this was accompanied by the selectivity for isopropyl alcohol dropping to 55 percent, and with a high production, about 45 percent, of diisopropyl ether.
German Patent Application No. 1 291 729 which discloses a processes employing a strongly acidic ion exchange resin as catalyst, teaches that these catalysts have a relatively short effective life time due to the hydrolysis of the aromatic bonded sulfonic acid groups, particularly at higher temperatures. The catalyst life generally is no longer than a few hundred on-stream hours. This reference discloses a way to prolong the catalyst life considerably by using exchange resins with aliphatic or non-aromatic bonded sulfonic acid groups. However, these resins have not been available commercially because their preparation is so complicated.
Another known method of mitigating the drawbacks of the prior art processes for the catalytic hydration of lower olefins with strongly acidic ion exchange resins in a trickle type reactor is by varying the operating conditions and using a different type of resin with aromatic bonded sulfonic acid groups. The journal "Industrial and Engineering Chemistry", Product Research and Development, Vol. 1 (1962), No. 4, pp. 296-302, published a paper which, in addition to showing the influence of pressure, temperature, throughput and other parameters upon IPA yield, selectivity, and space-time yield compares two commercial catalyst, i.e. "Amberlite IR-120" and "Amberlyst 15", of which the latter has a macroreticular structure and a particularly large specific surface area. The properties of these two synthetic resins are compared to each other in the "Journal of Polymer Science", Part C, 1967, pp. 1457-69, on page 1463. According to this paper, "IR-120" is of the gel type, has a specific surface area below 0.1 m.sup.2 /g, a pore radius which can hardly be measured, a porosity of 0.003 ml/ml resin, a water absorption capacity of 46 weight percent, and an ion exchange total capacity of 4.6 milliequivalents/g.
"Amberlyst 15" is a macroporous resin, has a specific surface area of 54,8 m.sup.2 /g, an average pore diameter of 288 A, a porosity of 0.367/ ml/ml resin, a water absorption capacity of 49 weight percent, and an ion exchange total capacity of 4.8 milliequivalents/g.
Although the two exchange resins differ quite considerably structurally, their performance in the hydration of propylene are reported to be rather similar as far as resistance of hydrolysis and catalyst performance are concerned (cf. the aforementioned paper "Ind. Eng. Chem.", page 297).